Shotgun gauge is determined by the size of a round ball of lead that would fit into the barrel.
A 12 gauge barrel is the size of 1/12 of a ball of lead.
A 16 gauge barrel is the size of 1/16 of a ball of lead.
A 20 gauge barrel is the size of 1/20 of a ball of lead.
A .410 bore is equivalent to a hypothetical 68 gauge shotgun.
(the bore size is 1/68 of a round ball of lead)
It has similar base dimensions to the .45 Colt revolver cartridge.
“A 12 gauge barrel is the size of 1/12 of a ball of lead.”
I think maybe you mean “...the size of 1/12 of a balled POUND of lead..?